There has been known a film forming apparatus for chemically forming a thin film on a semiconductor wafer (hereinafter, referred to as ‘wafer’ for simplicity). In such a film forming apparatus, the thin film is formed on the wafer by using a plasma or the like.
However, after forming the thin film on the wafer, there are found reaction by-products adhering on an inner wall of a chamber or the like. In case of forming the thin film on the wafer in the presence of the reaction by-products accumulated on the inner wall of the chamber or the like, the reaction by-products may then peel off therefrom to contaminate the wafer. Accordingly, there is a need to remove the reaction by-products adhered to the inner wall of the chamber or the like by regularly cleaning an interior of the chamber.
In cleaning the interior of the chamber, it is important to detect a proper end point of the cleaning in order to avoid an insufficient cleaning, a damage to the inner wall of the chamber or the like due to an excessive cleaning, and a waste of a cleaning gas. Currently, as for a method of detecting the end point of the cleaning, there is known a method of measuring a luminous intensity of a plasma by a spectrometer and then detecting the end point based on the luminous intensity.
Such a method, however, is problematic in that it requires a plasma to be generated to detect the end point of the cleaning, and consequently, is unusable if the cleaning is carried out without generating a plasma.